Cleaner fish and third-party punishment

Recently, a study of cleaner fish revealed how males will punish females for bad behavior—even when they seem to be bystanders, and are not personally affected by the females.

Nichola Raihani and colleagues describe how cleaner fish, Labroides dimidiatus, serve bigger fish in nature by eating tiny parasites off their bodies. The cleaner fish would rather take a bite of the bigger fish's mucous, but that offensive act causes the bigger fish to swim away.

The researchers found that male cleaner fish will punish their female partners if the females offend the larger fish by eating its mucous—even though the male cleaner fish are not directly harmed by this behavior. Their finding may help to shed light on the origins and evolution of more complex behavior in humans, researchers say.

In their experiment, Raihani and colleagues introduced a plate full of normal fish flake (which the fish will eat) and prawns (which the fish much prefer) to a tank with two cleaner fish. Then, they removed the plate whenever the fish ate a prawn.

The researchers quickly observed that male cleaner fish would punish females whenever the females would eat a prawn, causing the plate to be removed from the tank. They also found that, after being punished, the females were much less likely to eat prawn again.

This practice of "third-party punishment" allowed the males to eat more fish flake from the plate, and it indicates that male cleaner fish could get from more food in nature if they punish females for eating a bigger fish's mucous—even though the bigger fish are the true victims.